The present invention relates to charge coupled device transversal filters and more particularly to an improved electrode arrangement and sensing process in which tap outputs are generated at the time of transfer of charge packets between successive sense wells with all sensing being performed by a single electrode.
References known to the present applicants and believed to be relevant to the present invention include the article entitled "The Design and Operation of Practical Charge-Transfer Transversal Filters" by Baertsch, et al., IEEE Trans. Electron Devices, vol. ED-23, pp. 133-141, February 1976; the article entitled "Self-Contained Charge-Coupled Split-Electrode Filters Using a Novel Sensing Technique" by Sequin, et al., IEEE Journal of Solid State Circuits, vol. SC-12, No. 6, pp. 626-632, December 1977; and the article "Fully Integrated CTD Filter With Output Sensing" by Weckler, et al., printed in the Digest of Technical Papers, pp. 84 and 85, 1978 IEEE International Solid State Circuits Conference.
Both the Baertsch and Sequin references discuss conventional charge coupled device transversal filters employing split electrodes. In the devices taught by both of these references, weighting of the various signal taps is provided by the difference in length of the two portions of a single electrode. Such an arrangement has several basic problems both in processing and in signal recovery. This type of device is sensitive to mask alignment and other processing variables such as over etching of the gate patterns. Thus, normal variations in etching will have larger percentage effects on the small segment of a split electrode than it will have on the large segment, thus changing the apparent difference in length. Mask misalignment which reduces the effective length of one portion of a split electrode simultaneously increases the effective dimension of the other portion with the result that the ratio of sizes is affected by an even higher percentage. These two references also provide several different sensing amplifier schemes, all of which recognize that the output signals from the split sense amplifiers have large common mode components with the signal being only a small percentage of the common mode signal. As a result, amplifiers having extremely high common mode rejection must be employed or additional amplifiers must be employed to remove the common mode component of the output leaving only the actual signal component to be processed by a sensing amplifier. The amplifiers employed are generally of the integrating amplifier variety comprising an operational amplifier with an integrating feedback capacitor. These feedback capacitors must be large enough to absorb the large common mode signal and thus must be many times larger than would be required for capacitors used to detect only the desired signal portion of the output.
Whereas, in FIG. 9 of the Baertsch reference, multiple amplifiers are used to clamp the split electrode segments at a fixed voltage to avoid what is termed "cross talk" among the charge samples, additional difficulties with the feed-back capacitors are encountered. In particular, the clamping amplifiers must be provided with feed-back capacitors which are not only quite large, but are matched within a fraction of a percent since any difference appears as an error in the signal output which is the difference between the output of the two clamping amplifiers. It is well known that large capacitor values must be avoided in such integrated circuits and the need for very close matching of two large capacitors in the same circuit makes the arrangement basically impractical.
Other sensing schemes such as shown in FIG. 13 of Baertsch operate with a single amplifier, but face a common mode range problem and the cross talk problem mentioned above. The common mode signal detracts from signal range since actual signal is measured relative to some reference level. Again, the two capacitors provided with the amplifier must be closely matched to maintain resolution and must be large enough to minimize loss in charge handling capability, but not so large as to unduly increase insertion loss.
The Sequin reference provides some different sensing schemes, but they also require large capacitors to absorb the background charge. The Sequin arrangements are also rather complex.
The Weckler reference teaches an analog filter constructed with a bucket brigade type device, as opposed to a charge coupled device, in which only one portion of each split electrode is connected to a single sense line. As noted in this reference, this arrangement improves tap weight accuracy and eliminates large common mode signals since the signal differencing actually occurs within the single sense electrode. It is stated in this reference that it is possible, in a bucket brigade structure, to sense simultaneously the signal under both clock phases with a common sense line as shown in the figures of that article. The device further employs a single reset operational amplifier integrator for sensing displacement currents which flow in the sense gates and are summed in the output line. Weighting coefficients are implemented as the difference in areas of adjacent sense gates which correspond to the two different clock phases employed in transferring charge between the buckets of the bucket brigade device. The integrator is reset on alternate clock phases and is allowed to integrate during one of the two transitions between clock phases. Since the sense electrodes are reset at a time when signal charge is present and the same charge is present, under different electrodes, at the time a reading is taken, the background or common mode portion of the output is effectively eliminated and only the difference resulting from tap weights appears as the signal.